High availability. What does it exactly mean ?

Maybe like me you are used to read architecture documents that state that an application should have high availability. It’s not uncommon to read that an application should have four-nine availability (99,99%). But what does it really mean ? In the JBoss in Action book there is a very clear explanation by giving the allowed downtime per year. Here is the table :

Allowed downtime per year
99% 87.6 hours
99.9% 8.8 hours
99.99% 53 minutes
99.999% 5.3 minutes
99.9999% 31 seconds
99.99999% 3.1 seconds

3.1 seconds downtime per year, it’s really not a lot. So if you sign a contract with 99.99999% availability, make sure you have good lawyers around you.

One thought on “High availability. What does it exactly mean ?

  1. Hi, thanks much for that fun post. I had been hoping to add your Rss but unfortunately it never look like working for me. Do you have other people having one of these problem? I’ll return to their office in one day perhaps to update. Regardless thanks a whole lot again for the piece.

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